Thermionic generators convert heat energy to electric power. Most thermionic generators have a planar emitter and a planar collector which are separated by ceramic spacers that also seal the space between the emitter and collector from ambience. The sealed space may be a near vacuum or a low pressure plasma. The emitters are typically made of a refractory metal having a low Fermi level while the collectors are made of a metal having a relatively high Fermi level.
When sufficient heat is supplied to the emitter, high energy free electrons obtain enough energy to escape from the emitter surface. This phenomenon is known as thermionic emission. The energy required to force these free electrons from the emitter is referred to as the surface work or work function. These basic physical principles are discussed in detail in Direct Energy Conversion, 3rd edition, by Stanley W. Angrist.
The passing of free electrons to the collector manifests a flow of electric current. However, the emission of electrons produces a space charge in the space between the emitter and the collector which severely limits the efficiency of the generator. To overcome this problem a low pressure plasma is maintained within the space to limit the space charge produced. Nevertheless, this type of generator still does not produce electric power efficiently. Additionally, thermal instabilities of the planar components often causes them to warp, thus making it difficult to maintain a proper spacing between the emitter and collector. An improper spacing causes inefficiencies and creates a risk of system failure. For example, if the emitter warps towards the collector it can contact the collector and thereby cause an electric short. Conversely, if the emitter warps away from the collector the spacing increases thereby resulting in a decrease in electron flow.
Thermionic generators have been designed that have electron discharge pins extending outwardly from the planar emitters which terminate very close to the collector, as shown in FIG. 1. The presence of these pins increases the efficiency by which the electrons flow from the emitter to the collector. However, here too thermal instabilities cause difficulties in maintaining a proper spacing between the ends of the pins and the collector.
Thermionic generators have also been designed having elongated tubular emitters telescopically positioned within elongated tubular collectors as shown in U.S. Pat. No. 3,265,910. This configuration decreases the overall size of the generator. Here again, however, these generators suffer from the effects of thermal instabilities which may cause the elongated emitter and collector to warp.
Accordingly, it is seen that a need remains for a more efficient and thermally stable thermionic generator. It is to the provision of such therefore that the present invention is primarily directed.